The triangle ACD is a right angle, since it is a triangle in a semicircle.
Application of pythagoras theorem:
The theorem states that the square of the hypotenuse is equal to the square of the other two sides; hence:
[tex]CD^2=AC^2+AD^2[/tex]
solving for CD:
[tex]CD=\sqrt{9^2+4^4}=9.8[/tex]
ANSWER
CD = 9.8
